Pristine Fossil Reveals Unlikely Pair

The small but sharp-toothed Thrinaxodon probably spent much of its time dining on its Triassic cohabitants, but a study published today reports a pristine fossil of the meat-eater apparently peacefully sharing its burrow with a small amphibian – until they were both buried in a flood.

The researchers uncovered the odd couple through non-destructive imaging 0f a burrow cast from South Africa, where the animals appeared to have died together. In the image of the cast itself, along with the ghostly outlines of the animal skeletons you can see that layer 1 is the original bed of the burrow, and layers 2 and 3 correspond to subsequent “pulses” of the flooding event.

The two skeletons are remarkably complete and well-preserved (in the image above Thrinaxodon is shown in brown, and the amphibian in grey), and the artifact provides an excellent opportunity to study the interactions between two different species. Given Thrinaxodon‘s carnivorous ways, it may at first seem most likely that the amphibian was about to be eaten for lunch, but its undisturbed skeleton and lack of expected bite marks rule out this possibility, the authors write. They also conclude that the flood responsible for burying the animals couldn’t have randomly washed the amphibian into the burrow once the animals were already dead because the burrow’s opening was too small.

To find the most likely answer, the researchers turned to modern creatures for insight. They note that animals today will live in a burrow built by another species if it is abandoned, if they can chase away the host, or if the host tolerates their presence. The Thrinaxodon was still in the den, so neither of the first two possibilities seem to apply in this case, leaving the last option as the most likely. As strange as it may seem, it appears that for whatever reason the Thrinaxodon graciously tolerated its amphibian partner’s presence.

If you want to see more, this video shows how the authors virtually dissected the burrow using synchrotron scanning to create an exquisitely detailed reconstruction of the burrow’s contents without cracking it open.